The invention generally relates to four-terminal driver for switching power devices such as MOSFETs, thyristors and triacs, requiring floating drive circuit and large input current pulses. The driver is characterized by an extremely high switching speed and is intended to operate in systems requiring a constant or occasional switching, e.g. switching power supplies and switching power amplifiers.
Switching power devices are controlled by a relatively small voltage applied across a pair of terminals. However, the input current may be substantial. Some power devices require continuous large input current, e.g. single bipolar transistors. Other devices demand merely large initial current. For instance, thyristor (silicon controlled rectifier or SCR) and triac can be triggered by a low-level voltage applied between a cathode and gate. A pulse of the input current is injected thereinto as these devices are automatically turned off by reducing an anode voltage or reducing principal current below a hold current.
The gate impedance of a power MOSFET is a high resistance shunted by a large composite gate capacitance, including Miller effect capacitance. Therefore, the MOSFET requires fairly large input current to change the gate voltage. Due to a limited transconductance, the switching speed of the MOSFET depends upon the rate at which gate charge is supplied or removed. Consequently, the MOSFET driver should minimize any lead inductance by reducing loop areas. For instance, each inch of wire adds about 20 nanohenries of inductance.
The conventional drivers comprise complex current pulse drive circuits. The employment of transformers having a small turns ratio results in large currents which must be delivered to the primary winding. Moreover, numerous problems are introduced. For instance, transformers are bulky and expensive. Their high frequency performance is very poor. Snubber and clamp circuits are necessary to prevent forward and reverse voltage spikes, and reduce RFI. In another method an optocoupler is employed for controlling the driver. A supply voltage is applied thereto by a auxiliary power supply including an isolation transformer. This method results in a higher performance. However, parasitic capacitances introduced by the auxiliary power supply must be very small and the parts count and size are increased.